Laser firing apparatus for high efficiency solar cell and fabrication method thereof

ABSTRACT

Disclosed are a laser firing apparatus for a high efficiency solar cell including laser generating unit and a fabrication method thereof. The laser firing apparatus for a high efficiency solar cell includes at least one laser generating unit that irradiates a laser irradiation on to an electrode region formed on a semiconductor substrate for the solar cell and heat-treats the electrode region. In addition, the fabrication method of a solar cell includes forming an electrode material on a semiconductor substrate for the solar cell; and forming an electrode by heat treating the electrode material by laser irradiation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2008-0137176, filed on Dec. 30, 2008, in the Korean IntellectualProperty Office, the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The embodiments of the present invention relate to a laser firingapparatus for a high efficiency solar cell, and more specifically, to alaser firing apparatus for a high efficiency solar cell including atleast one laser generating unit that forms a front electrode unit byirradiating a laser to the front electrode unit formed on a frontsurface of a semiconductor substrate for the solar cell, and forms aback surface field (BSF) layer by irradiating a laser to a back metalpaste region formed on a back surface of the semiconductor substrate,and a fabrication method thereof.

2. Description of the Related Art

Upon classifying a solar cell based on a substrate material therefor,the solar cell may be further classified largely into three types, suchas a crystalline silicon based solar cell, an amorphous silicon basedsolar cell, and a compound semiconductor based solar cell. Further,types of the crystalline silicon based solar cell includes a singlecrystalline solar cell and a polycrystalline solar cell.

In a fabrication process of the silicon crystalline solar cell, a firingprocess performed by heating the silicon crystalline solar cell isessential. At this time, the firing process is generally performed by amethod that performs heat treatment for several minutes at a hightemperature by using a belt furnace. During performing of the firingprocess, a phenomenon occurs whereby lifetime of carriers is reduced,which has an effect on the efficiency of the solar cell. Therefore, inorder to maintain the lifetime of the carriers, a study on the methodthat minimizes the heat treatment time of the silicon substrate isurgently needed.

SUMMARY OF THE INVENTION

The embodiments of the present invention proposes to address theproblems in the related art as described above. It is an object of thepresent invention to provide a firing apparatus that can perform heattreatment of a solar cell within a short time.

In addition, it is another object of the present invention to provide afabrication method of a solar cell with increased efficiency by usingthe firing apparatus.

In order to achieve the above and other objects, there is provided alaser firing apparatus for a solar cell according to one aspect of thepresent invention including at least one laser generating unit thatirradiates a laser irradiation on to an electrode region formed on asemiconductor substrate for the solar cell and heat-treats the electroderegion.

In another aspect, the laser irradiation is in a line form.

In another aspect, the laser irradiation is in a line form whose outputlight is formed by a slit.

In another aspect, the laser firing apparatus for the solar cell furtherincludes a plurality of laser generating units disposed on a line.

In another aspect, the laser generating unit includes a first lasergenerating unit that is positioned to irradiate a front surface of thesemiconductor substrate and a second laser generating unit that ispositioned to irradiate a back surface of the semiconductor substrate,and the first laser generating unit and the second laser generating unitare disposed to be oppose each other.

In another aspect, the laser generating unit includes the first lasergenerating unit and the second laser generating unit that irradiate onesurface of the semiconductor substrate and are disposed to be parallelwith each other.

In another aspect, the laser firing apparatus for the solar cell furtherincludes a stage that seats the semiconductor substrate.

In another aspect, the stage is at least one belt type moving unit thatmoves the semiconductor substrate.

In another aspect, the at least one belt type moving unit includes afirst belt type moving unit and a second belt type moving unit.

In accordance with another aspect of the present invention, there isprovided a fabrication method of a solar cell, including forming anelectrode material on a semiconductor substrate for the solar cell; andforming an electrode by heat treating the electrode material by laserirradiation; wherein the electrode material comprising electrode paste,electrode ink and aerosol for electrode.

In another aspect, the electrode material is formed according to a frontelectrode pattern and is formed on the front surface of thesemiconductor substrate.

In another aspect, the material for the front electrode formed accordingto the front electrode pattern is heat-treated at a temperature of 600°C. to 1000° C. by the laser irradiation.

In another aspect, the electrode material is formed according the backelectrode pattern and is formed on the back surface of the semiconductorsubstrate.

In another aspect, the material for the back electrode formed accordingto the back electrode pattern is heat-treated at a temperature of 450°C. to 750° C. by the laser irradiation.

In another aspect, the material for the back electrode formed on theback surface of the semiconductor substrate is formed as the backelectrode by heat treatment and a back surface field (BSF) layer isformed at an interface between the back surface and back electrode ofthe semiconductor substrate.

In another aspect, the material for the front electrode and the materialfor the back electrode are simultaneously fired by simultaneous laserirradiations.

In another aspect, the semiconductor substrate is heat-treated by laserirradiations.

In another aspect, the semiconductor substrate is a p type impuritysemiconductor substrate or an n type impurity semiconductor substrate.

In another aspect, the fabrication method of a solar cell furthercomprises forming an antireflective layer on the front surface of thesemiconductor substrate prior to forming the electrode material.

In another aspect, the fabrication method of a solar cell furtherincludes forming a back passivation layer on the back surface of thesemiconductor substrate prior to forming the electrode material.

The heat treatment is performed in a short time by using the laserfiring apparatus of the present invention, such that the lifetime of thecarriers affecting the efficiency of the solar cell can be increased ascompared to the case of using the existing belt furnace.

In addition, with the fabrication method of the high efficiency solarcell according to the present invention, the solar cell with theincreased efficiency can be fabricated.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description of exampleembodiments given in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a cross-sectional view of a solar cell;

FIG. 2 is an exemplified diagram of a laser firing apparatus of a solarcell that includes a first laser generating unit and a second lasergenerating unit according to one embodiment of the present invention;

FIG. 3 is an exemplified diagram of a laser firing apparatus of a solarcell that includes one laser generating unit according to one embodimentof the present invention;

FIG. 4 is an exemplified diagram showing a laser in a line formaccording to one embodiment of the present invention;

FIG. 5 is an exemplified diagram showing a laser in a line form whoseoutput light is formed by a slit according to one embodiment of thepresent invention;

FIG. 6 is an exemplified diagram showing the time and the temperatureapplied according to high speed firing and existing firing; and

FIG. 7 is an exemplified diagram showing the measurement of the shortcurrent density (J_(sc)) and the open circuit voltage (V_(oc)) accordingto high speed firing-Solar cell and existing firing-Solar cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of the present invention will now be described morefully hereinafter with reference to the accompanying drawings forming apart of this specification wherein like reference characters designatecorresponding parts in the several views. In the embodiments of thepresent invention, detailed description of the functions andconfigurations that make the purport of the present inventionunnecessarily obscure are omitted.

In order to more clearly describe a function of a laser firing apparatusfor a solar cell according to embodiments of the present invention, aconventional fabrication method of a solar cell will be first describedfor contrast.

FIG. 1 is a cross-sectional view of a solar cell. Referring to FIG. 1, aconventional fabrication method of the solar cell will be brieflydescribed below. Thereafter, the solar cell of FIG. 1 will be referredto in describing a fabrication method of the solar cell according toembodiments of the present invention.

First, an n+ layer 102 is formed by diffusing phosphorous on a frontsurface of a p type impurity semiconductor substrate 101 and anantireflective layer 103 is then formed on the n+ layer 102. A metalpaste is subsequently screen-printed and dried on the front surface anda back surface of the substrate 101, and the substrate 101 is thenheat-treated in a belt furnace.

As a result, on the front surface of the substrate, silver (Ag)contained in the metal paste passes through the antireflective layer 103to contact the n+ layer 102, thereby forming a front electrode unit(fire-through contact) 104. In addition, on the back surface of thesubstrate, an Al containing paste 106 is diffused into the substrate toform a back surface field (BSF) layer 105 and form a back surfaceelectrode unit 107 together therewith.

When performing the heat treatment to form the BSF layer 105 andelectrode units 107, 107 of the solar cell according to a conventionalfabrication method, the time spent on the belt furnace is about 1 to 2minutes and the maximum temperature reached is about 800° C. In the heattreatment process according to the conventional fabrication method, alifetime of charge carriers is suddenly reduced. If the life time isreduced, a probability of recombining holes and electrons generated byreceiving light is large and thus, the efficiency of the solar cell isreduced.

Generally, when the metal paste containing the Ag disposed on the frontsurface and back surface of the substrate is subjected to a screenprinting and drying step in a predetermined pattern, a lifetime ofcharge carriers corresponds to about 18 to 20 μs. However, when beingsubjected to the heat treatment step of the belt furnace, the lifetimeis suddenly reduced to about 4 μs.

As described above, in order to prevent or reduce the phenomenon of thedecreased lifetime of the charge carriers, which affects the efficiencyof the solar cell due to the high-temperature heat treatment using thebelt furnace, in embodiments of the present invention, the solar cell isfabricated using a laser (laser beam or laser irradiation) that canlocally heat the semiconductor substrate and the heat treatment isperformed within a short time.

Generally, the conversion efficiency η of the solar cell refers to avalue that divides open circuit voltage Voc×short current densityJsc×fill factor (FF) by total light energy (device area S×intensity I oflight irradiated to a solar cell). Therefore, in order to increase theconversion energy η of the solar cell, it is important to make the shortcurrent density Jsc and/or the open circuit voltage Voc large. One ofthe factors affecting the values of the short current density Jsc and/orthe open circuit voltage Voc are a firing time of the solar cell.

As can be appreciated from FIG. 6, the firing process is performed at atemperature of about 800° C. during the fabrication process of the solarcell. At this time, the solar cell is fabricated by being divided intotwo cases, i.e., existing firing and high-speed firing. The existingfiring refers to the case of firing during a relatively long timeaccording to the existing method and the high-speed firing refers to acase of firing during a relatively shorter time as compared to theexisting method.

In order to review the effect on the conversion efficiency η of thesolar cell fabricated by changes in the firing time as described above,the short current density Jsc and the open circuit voltage Voc aremeasured. The measured results are shown in FIG. 7.

As can be appreciated from FIG. 7, the high-speed firing of the solarcell obtains the short current density Jsc and the open circuit voltageVoc, all of which have a higher value than the existing firing of thesolar cell. In other words, in order to obtain the solar cell having thehigh conversion efficiency η, a need exists for the fabrication methodof the solar cell by the high-speed firing.

Therefore, a fabrication method of the solar cell by the high-speedfiring using a laser (a laser beam or a laser irradiation) by the laserfiring apparatus for the high efficiency solar cell according to oneembodiment of the present invention will be described with reference toFIG. 2.

The laser firing apparatus for the solar cell of FIG. 2 includes a firstlaser generating unit 201, a second laser generating unit 202, a firstbelt type moving unit 203, and a second belt type moving unit 204. Asshown in FIG. 2, the first belt type moving unit 203 and the second belttype moving unit 204 are disposed to be separated by a predetermineddistance to allow a laser (a laser beam or a laser irradiation)generated in the second laser generating unit 202 to pass between thefirst and second belt type moving units 203 and 204 where they areseparated.

First, an n+ layer is formed on a front surface of a semiconductorsubstrate 205, and the semiconductor substrate 205 for the solar cell,on which a back metal paste region is formed on a back surface of thesubstrate, is seated on the first belt type moving unit 203. Theembodiment uses a p type impurity semiconductor substrate, but is notlimited thereto. Therefore, the embodiment can also use an n typeimpurity semiconductor substrate.

The semiconductor substrate 205 for the solar cell seated on the firstbelt type moving unit 203 is irradiated by a first laser (a first laserbeam or a first laser irradiation) and a second laser (a second laserbeam or a second laser irradiation) while moving to the second belt typemoving unit 204. The first laser is generated in a line form in thefirst laser generating unit 201 and the temperature of the semiconductorsubstrate 205 for the solar cell irradiated by the first laser reaches atemperature range of 600 to 1000° C. The first laser is irradiated onthe front electrode unit of the semiconductor substrate 205 for thesolar cell to fire and allow the front electrode unit to contact the n+layer.

In addition, the second laser is generated in a line form in the secondlaser generating unit 202 and the temperature of the semiconductorsubstrate for the solar cell irradiated by the second laser reaches atemperature range of 450 to 750° C. The second laser is irradiated onthe back metal paste region of the semiconductor substrate for the solarcell, thereby forming a back surface field (BSF) layer. The line form ofoutput light of the first and/or second laser may be formed by a slit,for example, the slit containing panel.

As such, according to the embodiment of the present invention, only thefront electrode unit and/or the back metal paste region of thesemiconductor substrate for the solar cell are locally heated by atleast one laser in a line form and heat-treated in a short time, therebymaking it possible to increase (maintain or not reduce) the lifetime ofthe carriers. As described above, when the lifetime is increased(maintained or not reduced), a probability of recombining holes andelectrons is reduced, thereby making it possible to fabricate the solarcell with the increased efficiency.

When the lifetime is about 4 μs upon fabricating the solar cell throughthe heat treatment process of the belt furnace by the existing method,the theoretical maximum Jsc value, that is verified by computersimulation, is only 92%. However, when fabricating the solar cell usingthe laser firing apparatus for the high efficiency solar cell, thelifetime is 10 μs or more and the theoretical maximum Jsc value, whichis verified by computer simulation, can reach up to 97% Jsc or more,thereby making it possible to fabricate the solar cell with theincreased efficiency.

The laser firing apparatus for the solar cell may further include ablowing system 206 and/or an evacuation system 207. The blowing system206 plays a role of blowing air, in order to discharge a fume generatedwhen a laser is irradiated on the front electrode unit away from thesolar cell. In addition, the evacuation system 207 plays a role ofabsorbing or sucking the fume, in order to discharge the fume away fromthe solar cell quickly.

As shown in FIG. 2, in the fabrication method of the solar cellaccording to one embodiment of the present invention, the first lasergenerating unit 201 is positioned to irradiate the front surface of thesemiconductor substrate and the second laser generating unit 202 ispositioned to irradiate the back surface of the semiconductor substratefor the high efficiency solar cell, such that they may be disposed to beopposed to each other.

However, the positions of the first laser generating unit 201 and thesecond laser generating unit 202 are not necessarily limited to theabove-mentioned positions. In other words, the first laser generatingunit 201 and the second laser generating unit 202 may be arranged on aline. Both the first laser generating unit 201 and the second lasergenerating unit 202 may be positioned to irradiate the front surface ofthe semiconductor substrate 101 or the back surface of the semiconductorsubstrate 101, such that they may be disposed in parallel with eachother.

In addition, in the above-mentioned embodiment, the laser irradiated tothe front electrode unit and the back metal paste region is irradiatedfrom separate laser generating units, that is, the first lasergenerating unit and the second laser generating unit, respectively, butis not limited thereto. In other words, as can be appreciated from FIG.3, the laser irradiated to the front electrode unit and the back metalpaste region may be generated from the same laser generating unit 301.

When the laser irradiated to the front electrode unit and the back metalpaste region is generated from the same laser generating unit 301, a Ptype semiconductor impurity substrate 305 may be moved by the belt typemoving unit at least twice in the firing process of the front electrodeunit and the process of forming the BSF layer respectively in order tofabricate the high efficiency solar cell.

FIG. 4 is an exemplified diagram showing a laser in a line formaccording to one embodiment of the present invention and FIG. 5 is anexemplified diagram showing a laser in a line form whose output light isformed by a slit according to one embodiment of the present invention.

FIG. 4 shows a plurality of laser generating units 401 that are arrangedalong a line to irradiate a plurality of lasers in a line form, and FIG.5 shows a laser generated by one laser generating unit 501 irradiatesthe laser in a line form, whose output light is formed (or guided) by aslit 502, on a semiconductor substrate 503.

According to another embodiment of the present invention, a fabricationmethod of a solar cell in another form by the laser firing apparatus forthe high efficiency solar cell will be described. In other words,referring to FIG. 2, a fabrication method of a solar cell including thefront electrode unit and the rear electrode unit using the laser firingapparatus for the high efficiency solar cell according to the presentinvention will be described.

First, an antireflective layer is formed on an upper portion of thefront surface of the p type impurity semiconductor substrate.Thereafter, the n+ layer and the front metal paste region aresequentially formed, and the back passivation layer is formed on theupper portion of the back surface of the substrate. Then, thesemiconductor substrate for the solar cell, on which the back metalpaste region is formed, is seated on the first belt type moving unit.The present embodiment uses the p type impurity semiconductor substrate,but is not limited thereto. Therefore, the present embodiment can alsouse the n type impurity semiconductor substrate.

The semiconductor substrate for the solar cell seated on the first belttype moving unit is irradiated by the first laser and the second laserwhile moving to the second belt type moving unit. The first laser, whichis generated in a line form by the first laser generating unit, isirradiated on the front electrode unit of the semiconductor substratefor the solar cell to fire and allow the front electrode unit to contactthe n+ layer. In addition, the second laser, which is generated in aline form by the second laser generating unit, is irradiated on the backmetal paste region and back electrode unit of the semiconductorsubstrate for the solar cell to form the back surface field (BSF) layerto fire and allow the back electrode unit to contact the semiconductorsubstrate. The laser in a line form whose output light is formed by aslit may be used.

The laser firing apparatus for the solar cell further includes a blowingsystem or an evacuation system and when a laser is irradiated, canrapidly discharge a fume occurring in the semiconductor substrate awayfrom the solar cell.

By being subject to the above processes, the fabrication of the solarcell where the antireflective layer and the front electrode unit areformed on the semiconductor substrate and the back surface field (BSF)layer, the back passivation layer, and the back electrode unit aresequentially formed is completed.

In one embodiment, the laser formed by the first laser generating unitand the laser formed by the second laser generating unit aresimultaneously irradiated, thereby simultaneously forming the frontelectrode unit and back electrode unit of the solar cell. However, thelaser generated from the first laser generating unit and the lasergenerated from the second laser generating unit need not be irradiatedat the same time. Thus, they may be irradiated at different times suchthat the front electrode unit and back electrode unit of the solar cellare not simultaneously formed.

In addition, in one embodiment, the first laser generating unit and thesecond laser generating unit are disposed at a position opposite to eachother, but are not limited to the position. In other words, the firstlaser generating unit and the second laser generating unit are arrangedon a line such that the laser generated from the first laser generatingunit and the laser generated from the second laser generating unit areirradiated at different times, and the front electrode unit and backelectrode unit of the solar cell are not simultaneously fired.

Although the present invention has been described in detail withreference to example embodiments, it will be understood by those skilledin the art that various modifications and equivalents can be madewithout departing from the spirit and scope of the present invention, asset forth in the appended claims. Also, the substances of eachconstituent features explained in the specification can be easilyselected and processed by those skilled in the art from the well-knownvarious substances. Also, those skilled in the art can remove a part ofthe constituent features as described in the specification withoutdeterioration of performance or can add constituent features forimproving the performance. Furthermore, those skilled in the art canchange the order to methodic steps explained in the specificationaccording to environments of processes or equipments. Thus, it isintended that embodiments of the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A laser firing apparatus for a solar cell, comprising: at least onelaser generating unit that irradiates a laser irradiation on to anelectrode region formed on a semiconductor substrate for the solar celland heat-treats the electrode region.
 2. The laser firing apparatus forthe solar cell according to claim 1, wherein the laser irradiation is ina line form.
 3. The laser firing apparatus for the solar cell accordingto claim 1, wherein the laser irradiation is in a line form, and theline form is formed by a slit.
 4. The laser firing apparatus for thesolar cell according to claim 1, wherein the at least one lasergenerating unit further comprises a plurality of laser generating unitsdisposed on a line.
 5. The laser firing apparatus for the solar cellaccording to claim 1, wherein the at least one laser generating unitincludes a first laser generating unit that is positioned to irradiate afront surface of the semiconductor substrate and a second lasergenerating unit that is positioned to irradiate a back surface of thesemiconductor substrate and the first laser generating unit and thesecond laser generating unit are disposed to oppose each other.
 6. Thelaser firing apparatus for the solar cell according to claim 1, whereinthe at least one laser generating unit includes a first laser generatingunit a the second laser generating unit that are positioned to irradiateone surface of the semiconductor substrate and are disposed to beparallel with each other.
 7. The laser firing apparatus for the solarcell according to claim 1, further comprising a stage used to seat thesemiconductor substrate.
 8. The laser firing apparatus for the solarcell according to claim 7, wherein the stage is at least one belt typemoving unit that moves the semiconductor substrate.
 9. The laser firingapparatus for the solar cell according to claim 8, wherein the at leastone belt type moving unit includes a first belt type moving unit and asecond belt type moving unit.
 10. A fabrication method of a solar cell,comprising: forming an electrode material on a semiconductor substratefor the solar cell; and forming an electrode by heat treating theelectrode material by laser irradiation; wherein the electrode materialcomprising electrode paste, electrode ink and aerosol for electrode. 11.The fabrication method of the solar cell according to claim 10, whereinthe electrode material is formed according to a front electrode patternand is formed on the front surface of the semiconductor substrate. 12.The fabrication method of the solar cell according to claim 11, whereinthe material for the front electrode is heat-treated at a temperature of600° C. to 1000° C. by the laser irradiation.
 13. The fabrication methodof the solar cell according to claim 10, wherein the electrode materialis formed according a back electrode pattern and is formed on the backsurface of the semiconductor substrate.
 14. The fabrication method ofthe solar cell according to claim 13, wherein the material for the backelectrode formed according to the back electrode pattern is heat-treatedat a temperature of 450° C. to 750° C. by the laser irradiation.
 15. Thefabrication method of the solar cell according to claim 13, wherein thematerial for the back electrode formed on the back surface of thesemiconductor substrate is formed into the back electrode by heattreatment, and a back surface field (BSF) layer is formed at aninterface between the back surface and back electrode of thesemiconductor substrate.
 16. The fabrication method of the solar cellaccording to claim 11, wherein the material for the front electrode andthe material for the back electrode are simultaneously fired bysimultaneous laser irradiations.
 17. The fabrication method of the solarcell according to claim 13, wherein the material for the front electrodeand the material for the back electrode are simultaneously fired bysimultaneous laser irradiations.
 18. The fabrication method of the solarcell according to claim 10, wherein the semiconductor substrate isheat-treated by laser irradiations.
 19. The fabrication method of thesolar cell according to claim 10, wherein the semiconductor substrate isa p type impurity semiconductor substrate or an n type impuritysemiconductor substrate.
 20. The fabrication method of the solar cellaccording to claim 10, further comprising forming an antireflectivelayer on a front surface of the semiconductor substrate prior to formingthe electrode material.
 21. The fabrication method of the solar cellaccording to claim 10, further comprising forming a back passivationlayer on the back surface of the semiconductor substrate prior toforming the electrode material.